The invention relates to an article that includes a silicon-containing substrate and an intermediate layer/coating with an external barrier coating such as a protective environmental/thermal barrier coating (E/TBC).
Silicon-containing substrates have been proposed for structures used in high temperature applications, such as in heat exchangers and advanced internal combustion engines. Silicon-containing substrates are also used in gas turbine engines. Higher operating temperatures increase the efficiency of gas turbine engines. Silicon-based composite ceramics have been proposed as materials for applications in combustors for supersonic commercial airplanes. However, in many applications involving water-containing environments, a silicon-based substrate will recede and lose mass because of the formation of volatile species, such as silicon hydroxide [Si(OH)4]. The recession rate due to the volatilization or corrosion is often unacceptably high so that an external barrier coating such as an environmental/thermal barrier coating (E/TBC) with high resistance to such environments is required.
The external barrier coating can be an environmental/thermal barrier coating (E/TBC) that comprises a chemically stabilized zirconia, such as yttria stabilized zirconia. These coatings are capable of preventing the substrate materials from being in direct contact with environmental oxygen that diffuses through the coatings fairly rapidly and reaches the underlying silicon-containing substrate. Oxidation of the silicon-containing substrate involves the formation of various gaseous products. For example, the following equations demonstrate the attack on silicon carbide (SiC) and silicon nitride (Si3N4):
SiC(s)+O2(g)xe2x86x92SiO2(s)+COx(g) (x=1,2)
Si3N4(s)+O2(g)xe2x86x92SiO2(s)+NOx(g) (x=0.5xe2x88x923)
The form of the gaseous products is dependent on the oxygen partial pressure in the system. These gaseous species have low solubility and diffusivity in silica (SiO2) and in other oxides, which causes them to be trapped at the external coating/substrate interface to form voids. The pressure of the gases in the voids can be sufficiently high at elevated temperatures to cause bursting. Voids can also interconnect to form large unbounded interfacial regions that result in coating spallation.
Thus, there is a need to prevent formation of gaseous oxidation products at an interface region between an environmental/thermal barrier coating (E/TBC) and a silicon-based substrate.
The present invention provides an article that prevents or substantially diminishes the formation of gaseous products at a coating-substrate interface. The invention is an article that comprises a silicon-containing substrate and at least one external environmental/thermal barrier coating. The external environmental/thermal barrier coating(s) is permeable to diffusion of environmental oxidant and the substrate is oxidizable by reaction with the oxidant to form at least one gaseous product. The article comprises an intermediate layer/coating between the silicon-containing substrate and the external environmental/thermal barrier coating(s) that is oxidizable to a nongaseous product by reaction with the oxidant in preference to reaction of the silicon-containing substrate with the oxidant.
In another aspect, the invention relates to a method of forming an article, comprising forming a silicon-containing substrate that is oxidizable by reaction with oxidant to form at least one gaseous product and applying an intermediate layer/coating onto the silicon-containing substrate, wherein the intermediate layer/coating is oxidizable to form a nongaseous product by reaction with the oxidant in preference to reaction of the silicon-containing substrate with the oxidant.